Electro-acoustic transducer

ABSTRACT

An electro-acoustic transducer comprises an enclosure of closed form containing a plurality of piezo-electric motors, each connected to the enclosure to excite the same, the motors having no mechanical connection between one another capable of transmitting static or dynamic forces. The transducer has two acoustic frequency ranges which are independent of the value of the static pressure prevailing on the outside of the transducer, one frequency range being due to the resonance of the envelope, and the other to the resonance of the motors.

0 UHlted States Patent [191 m1 3,731,267 Brandt et al. v 1 May 1, 1973 54 ELECTROACOUSTIC TRANSDUCER 2,632,634 3 1953 Williams. .3401 x 7 ,74 l2 l9 6 S i. ....i [76] Inventors: Otto Brandt, Villa Aurora-Quartier 4 2 les Faysses, 83 Le Brusc; Bernard Tocquet, 2, Hameflu Pmlssaraque, Primary Examiner-Benjamin A. Borchelt 83 Sanary S/Mer both of France Assistant Examiner-l-l. J. Tudor [22] Filed; Jam 4 971 Attorney-Waters, Roditi, Schwartz & Nissen 211 Appl. No.: 103,666 ABSTRACT An electro-acoustic transducer comprises an enclo- [52] [1.8. CI ..340/9, BIO/8.4 sure of closed form containing a plurality of piezm II. C.- ..04'1 l t i motors. h connected t th enclosure t [58] Field of Search ..3l0l8.4,8.7; 340/8, excite the same, the motors having no mechanical 340/9 10 connection between one another capable of trans mitting static or dynamic forces. The transducer has nem'ences Cited two acoustic frequency ranges which are independent of the value of the static pressure prevailing on the UNlTED STATES PATENTS outside of the transducer, one frequency range being 3,525,071 8/1970 Massa, .lr ..340 9 due to the resonance of the envelo e, and the other to 2,497,680 2/1950 Massa the resonance of the motors, 2,838,850 6/1958 Stephenson et al... 3,150,347 9/1964 Hanish ..340/l0 3 Claims, 2 Drawing Figures Patented May 1, 1973 F/GJ laza ELECTRO-ACOUSTIC TRANSDUCER BRIEF SUMMARY OF THE INVENTION The invention relates to electro-acoustic transducers of the piezo-electric type.

The known electro-acoustic transducers of the piezoelectric type are very sensitive to forces due to the ambient static pressure, especially if these forces are not regularly distributed among the piezo-electric elemerits. This phenomenon is particularly troublesome in the case of transducers employed at considerable depths in water, in which the forces due to the hydrostatic pressure increase linearly with the depth of immersion. The electric-acoustic characteristics of the transducer are then modified in very large proportions which'it is difficult to foresee.

These transducers only have an appropriate output in a single, more or less narrow frequency band around their fundamental resonance frequency.

The latter frequency depends upon the dimensions and the structure of the transducers. For luw frequencies, for example, of several kilocycles per second, and considerable radiation acoustic powers, the overall dimensions of the present transducers and their weight are considerable.

The present invention contemplates a transducer obtained by the combination of known elements, which are assembled in a novel manner so as to obviate the aforesaid disadvantages.

According to the invention the radiating surface of the transducers is constituted by all or part of an envelope of any form, which is closed, which has appropriate rigidity and which is excited by one or more piezo-electric motors disposed in the interior, there being no mechanical connection between the said motors, when there is a plurality of them, except for the said envelope. Each motor is mechanically connected to the envelope only by a single portion of its surface. Due to the rigidity of the envelope, it protects the motors from the action of the external static pressure.

This same envelope also has, as a result of its shape and of its construction, a characteristic resonance frequency, which will be denoted by f and which is different and generally lower than that of the piezo-electric motors, which is denoted by f,,,. Consequently, the transducer according to the present invention may be regarded as a system of two coupled mechanical oscillations and accordingly it has two main resonance frequencies, of which one, f is lower than f while the other, f is in the vicinity of f,,.. By selection of the values of f, and f,,., the number of motors n and the distribution of the masses and of the elasticities, it is possible to vary within very wide limits the frequencies f and f the corresponding values of the quality factor Q and, more generally, all the electro-acoustic characteristics of the transducer.

The existence of a resonance frequency below that of the components of the system result in a reduction in weight and in overall size. I

The static pressure prevailing within the envelope of the transducer, which is independent of that outside it, as is clearly apparent from what has been stated in the foregoing, may be brought to any value, and particularly above atmospheric pressure, so that it is possible either to extend the field of application of the transducer or to lighten its structure and, if necessary, to improve its performances.

The transducers according to the present invention, which are intended particularly for the transmission of acoustic vibrations, may also serve (in accordance with the laws of reciprocity) for the reception of such vibrations in the medium surrounding them, by converting them into electric oscillations.

The following description and the accompanying drawings, illustrating two particular but non-exclusive embodiments and an application referred to without any limiting character, will enable the invention to be more readily understood. It relates to a transducer having reduced overall size which radiates under a high hydrostatic pressure, a high acoustic power whose level is constant in all'directions in a horizontal plane.

BRIEF SUMMARY OF THE INVENTION FIG. 1 is a perspective view of this transducer, from which the two side covers have been removed in order to show the interior of the transducer; and

FIG. 2 illustrates an equivalent circuit of the transducers employed for the theoretical calculation of the characteristics of the transducer.

DETAILED DESCRIPTION In a first embodiment, the transducer comprises an annular envelope 1 of steel, the side covers of the transducer being removed to show its interior. The external diameter of the envelope is 20 cm, its internal diameter is 17 cm and its depth is 4.2 cm. The transducer is intended to transmit acoustic vibrations to the surrounding medium, and it is excited by eight piezo-electric motors 2 disposed radially in the envelope and secured to the internal face thereof.

These motors have no mechanical connection apart from that with the envelope 1. Each of the motors is provided with a counter-weight 3 of grams; the active portion is composed of two piezo-electric ceramic elements 4, each having a thickness of 1 cm, an external diameter of 3.6 cm and an internal diameter of 1.2 cm. They may be subjected to an RMS voltage of 2,500 volts.

The two side covers which are inactive from an acoustic viewpoint complete the enclosure which protects the motors against the external pressure.

The transducer thus constituted has two resonant frequencies of 6 and 20 kilocycles per second, while the characteristic resonant frequency of the envelope or ring 1 alone is 8.5 kilocycles per second and that of the motors alone is 24 kilocycles per second. The quality factor for the lowest frequency (6 kilo-cycles per second) is 14, the efficiency 0.84 and the radiated acoustic power watts for the excitation voltage of 2,500 volts. The equivalent parallel resistance R, is 27 kilohms. These characteristics are substantially independent of the pressure exerted on the external face of the ring, which may reach 400 bars.

According to a second embodiment there is shown the capability by which it is possible to vary the electroacoustic characteristics of the transducers according to the present invention.

The second embodiment is a variant of the preceding embodiment, the arrangement of the component members being similar. The ring has an external diameter of 33 cm and a thickness of 4 cm. It consists of A U 4 G. The number of motors and the counterweight is 596 grams, the active portion being constituted by four piezo-electric ceramic elements consisting of lead zirconate and having an external diameter of 3.6 cm, an internal diameter of 1.2 cm and a thickness of 1 cm. The low frequency of the transducer is 2.5 kc/s and the high frequency is 9.5 kc/s, while the frequency of the motor alone and of the ring alone are S and kc/s respectively. The parallel equivalent resistance is 7,560 ohms and the efficiency is 97 percent. The radiated acoustic power reaches 4 kilowatts for an excitation voltage of 5,500 volts. The quality factor Q for the low frequency is 4.8. The ring and its covers are capable of withstanding a pressure of 200 bars.

The value of the resonant frequencies of the transducers according to the present invention may be obtained by the following calculation: let f,, be the characteristic resonance frequency of the radiation part of the envelope; in the particular case ofa ring a, fa

where E,, is the elasticity of the motor, M is the fraction obtained by equalled division of the mass of the vibration part of the envelope among all the motors and M is the value of the counter-weight. Finally, the ring and the coupled motors may be representedby the equivalent circuit as shown in FIG. 2, in which there appear the above-defined quantities and in which, in addition, F which is the excitation force in the active part (piezo-electric elements), V, which is the speed of oscillation of the mass M and E which is the elasticity of the ring, are auxiliary factors in the calculation.

The frequencies f and f are obtained by resolving the system of equations corresponding to this circuit. By taking f,, a f,,, there are obtained solutions of the form:

It will be obvious that the nature, the form and the physical properties of the members of which this transducer is composed may be modified without affecting the principle of the invention.

manner to at least a part of said envelope whose characteristic resonance ,{frequency is fa, said transducer having two acoustic resonance frequenciesf and f different from fa and fm.

2. An electro-acoustic transducer according to claim' 1 in which the frequency fa is lower than fm and f is lower than fa,f being in the vicinity of fm.

3. An elefctro-acoustic transducer as claimed in claim 1 wherein the envelope comprises a ring closed by two rigid covers and, in the interior of said envelope, a plurality of said electro-acoustic motors each having an axis of revolution, said motors being secured to the internal face of said ring in vibration-transmitting manner, each said axis being disposed radially and said motors comprising piezo-electric rings and a counterweight placed at the end of each motor nearer the center of the ring, said counter-weights being shaped in the form of conical tips so that the motors can be placed in spaced apart relationship. 

1. An electro-acoustic transducer for producing or receiving acoustic vibrations in deep water, said transducer comprising a rigid, closed, envelope filled with air, and at least one electro-acoustic motor disposed in the interior of said envelope, said motor having a characteristic resonance frequency fm, and being mechanically connected in vibration-transmitting manner to at least a part of said envelope whose characteristic resonance frequency is fa, said transducer having two acoustic resonance frequencies f1 and f2 different from fa and fm.
 2. An electro-acoustic transducer according to claim 1 in which the frequency fa is lower than fm and f1 is lower than fa, f2 being in the vicinity of fm.
 3. An electro-acoustic transducer as claimed in claim 1 wherein the envelope comprises a ring closed by two rigid covers and, in the interior of said envelope, a plurality of said electro-acoustic motors each having an axis of revolution, said motors being secured to the internal face of said ring in vibration-transmitting manner, each said axis being disposed radially and said motors comprising piezo-electric rings and a counter-weight placed at the end of each motor nearer the center of the ring, said counter-weights being shaped in the form of conical tips so that the motors can be placed in spaced apart relationship. 